The Advanced Very High Resolution Radiometer (AVHRR) is a four or five channel (depending on the model) scanner, sensing in the visible, near-infrared, and thermal infrared portions of the electromagnetic spectrum. This sensor is carried onboard the National Oceanic and Atmospheric Administration's (NOAA's) polar orbiting satellites. The satellites orbit the Earth 14 times each day at a height of 833km. Each pass of the satellite covers a 2400km wide swath. At nadir the pixel size is approximately 1.1km. There are four classes of AVHRR data: High Resolution Picture Transmission (HRPT) data are full-resolution (1 km) data received directly in real-time by ground stations; Global Area Coverage (GAC) are sampled on-board to represent a 4.4 km pixel, allowing daily global coverage to be systematically stored and played back to NOAA ground stations; Local Area Coverage (LAC) are 1 km data recorded on-board for later replay to the NOAA ground stations and Automatic Picture Transmission (APT) is an analog derivative of HRPT data transmitted at a lower resolution and high power for low-cost VHF ground stations.